(1) Field of the Invention
The present invention generally relates to a vehicle body vibration control apparatus, and more particularly to a vehicle body vibration control apparatus for decreasing a pitching vibration of a vehicle body due to a rough surface of a road.
(2) Description of the Prior Art
Conventionally, there is known an apparatus for controlling the damping force of a shock absorber provided between a vehicle body and each wheel so that a vibration of a vehicle body due to a rough road surface is suppressed in an early stage of the vehicle body vibration (see Japanese Laid-Open Patent Application No. 62-166104). The proposed apparatus is directed to improving ride comfort and suppressing the vehicle body vibration in an early stage thereof. For these purposes, the damping force of each of the shock absorbers is altered to increase after the elapse of a predetermined delay time That is, the damping force is not changed to increase immediately after a vehicle body vibration takes place The predetermined delay time is set equal to or less than half of the first one-cycle of the vehicle body vibration.
It will be noted that there is a possibility that the phase of the vehicle body vibration on the front-wheel side is different from that on the rear-wheel side due to the relationship between a vehicle speed and a wheel base In some cases, the phase of the vehicle body vibration on the front-wheel side is approximately 180.degree. out of phase with that on the rear-wheel side. In such cases, the vehicle body does not have a simple bouncing vibration (a vertical movement of the entire vehicle body) but a pitching vibration. Pitching vibrations of the vehicle body give a person in the vehicle body a much more uncomfortable ride than given by bouncing vibrations.
The apparatus disclosed in the aforementioned Japanese application is capable of definitely suppressing the vehicle body vibration in an early stage thereof when the vibration is a bouncing vibration. However, the apparatus cannot effectively eliminate problems arising from a pitching vibration which causes the phase difference between the front-wheel-side vibration and the rear-wheel-side vibration.
There is also known an apparatus in which the damping force of each shock absorber and the spring constant of each suspension on the front-wheel side is controlled independently of those on the rear-wheel side so that natural vibrations of the vehicle body having different frequencies on the front and rear-wheel sides can be appropriately suppressed (see Japanese Laid-Open Utility Model Application No. 63-4808). However, this apparatus controls the damping force irrespective of the phase difference between the front-wheel-side vibration and the rear-wheel-side vibration. Thus, it is impossible to effectively suppress a pitching vibration any time it occurs until the vehicle body vibration is completely suppressed.
As has been described above, the conventional vehicle body vibration control apparatuses do not have any means for handling pitching vibrations. Thus, there is room for further improvements in a vehicle body vibration control apparatus which will lead to greater ride comfort.